3.4 Manual Control of GEM Choppers

The GEM choppers are described in section 2.3

The electronics for controlling the GEM choppers are located in the electronics half of the GEM cabin.

GEM Manual Disc Chopper (9.75m)

The opening angle of this chopper is fixed, and the user can only stop and start this chopper, or change its phase.

You can start (or stop) this chopper by pressing the green (or red) button on the “stop/start cortina” card in the “GEM Disc” electronics rack. 

The standard setting for the phase delay is 19493 micro-seconds. This delay may be changed by moving the setting on the thumbwheels in the “Phase Delay” card, and then pressing the small white button below the thumbwheels. For this to work, the small silver toggle-switch must be set to MAN, not COMP, or otherwise the chopper will be under computer control, not manual control.

See section 4.10 for information on computer control of the disc choppers.

GEM Indexing Disc Chopper (6.5m)

You can stop and start this chopper, or change the phase, using the same procedures as for the Manual Disc Chopper. The standard setting for the phase delay is 19302 micro-seconds.

It is also possible to change the opening aperture of the Indexing Disc Chopper, which is usually set to 112°. To change this opening angle, first stop the chopper. Then use the “digit select” and “digit adjust” buttons on the “Chopper Indexing Controller” to select your new opening angle. Then press start on the “Chopper Indexing Controller”. When the new opening angle has been set, you may re-start the chopper.

See section 4.10 for information on computer control of the disc choppers.

GEM Nimonic Chopper 

This chopper is usually stopped for experiments requiring high Q or low d-spacing data. It is of greatest benefit for the reduction of background from massive pieces of sample environment equipment. To start (or stop) the nimonic chopper, press the green (or red) buttons on the “stop/start indramat” card in the “GEM Nimonic” electronics rack. If you stop the nimonic chopper then it should be removed from the chain of vetoes cables.

Chopper Vetoes

Usually the instrument is set up (using the change command) so that neutron data are only acquired if the choppers are running correctly. This is achieved by a system of vetoes. If a chopper is not being used then it should be removed from the chain of veto cables on the chopper electronics so that it does not prevent data acquisition.

GEM Chopper Power Supplies

These are located on the ground floor level.

Last updated on 25 Feb 2003 by Alex Hannon (a.c.hannon@rl.ac.uk)